Method for determining the top dead centre of an internal combustion engine

ABSTRACT

Direct TDC determinations on internal combustion engines involve much effort and are relatively inaccurate, above all on multicylinder engines. Known TDC determinations from the pressure curve inside the cylinder, using the adiabatic equation of state, call for considerable calculation and yet do not take into account the actual operating conditions of the engine.  
     With the new procedure, after calibrating the rotary position transducer employed for direct TDC determination, the pressure curve inside the cylinder and the directly measured TDC timing in various operating states and if necessary different engine-specific parameters are recorded simultaneously, correlated as discrete value pairs and stored as a data set.  
     With one part of the data set a knowledge-based system is trained and varied till the TDC timing output for all operating states and engine types considered agrees with a TDC time previously measured directly, within the desired accuracy.  
     To detect the TDC from the pressure curve inside the cylinder with unkown and possibly different engine types, measured pressure values are put into the knowledge-based system unaltered after training, at whose output the required TDC is given.

[0001] The invention concerns a method for exactly determining the topdead centre (TDC) of an internal combustion engine from the pressurecurve inside the cylinder. The top dead centre (TDC), more exactly thepoint-at the end of the compression phase, is known to be an importantvalue in the cycle of a reciprocating combustion engine, as forcalculating the cylinder power output for example. Determining itdirectly involves much effort and is relatively inaccurate, especiallyon multicylinder engines. Accordingly determining the TDC from thepressure curve inside the cylinder has already been suggested—by K.Wehner, “Bestimmung des dynamischen OT aus dem Zylinderinnendruckverlaufvon Verbrennungsmotoren”, IBZ e.V.; (Innovation and Education Centre,registered association in Germany; published in 1996). This known methodcompares the pressure curve inside the cylinder calculated with theequation of the ideal adiabatic changes of state with the measuredcurve; the parameters of the calculated curve are then variediteratively till adequate agreement with the measured curve is attained.As the final step the TDC calculated with the approximation procedure isthen taken over into the measured curve. This method involves aconsiderable amount of calculation firstly, while secondly it does nottake into account the actual conditions during the operation of theengine, such as heat losses of the gas, gas losses (i.e. leakages) anddeformations of the engine. Though these errors can be compensated inpart with empirical values or mathematical models, this then calls forstill more calculation.

[0002] The purpose of the invention therefore is to provide a method fordetermining the TDC from the pressure curve inside the cylinder,demanding relatively little calculation and taking into account theactual engine behaviour.

[0003] According to the invention this task is fulfilled by the featuresof claim 1.

[0004] Provided there is a properly trained knowledge-based system, thenTDC detection and possibly different engine-specific parameters demandonly one measurement of the pressure curve inside the cylinder and inputof the pressure values into the knowledge-based system, which haslearned to allocate the right TDC to them.

[0005] The subclaims cover advantageous developments of the invention.Thus the structure of the knowledge-based system and the training effortcan be reduced with engines of one type, such as large-bore dieselengines having the same engine-specific parameters, by using only thepressure curve inside the cylinder for TDC detection in the trainedsystem. To verify whether the knowledge-based system is adequatelytrained it has proved efficacious to subdivide the quantity ofcorrelated cylinder pressure and TDC time pairs from direct measurementinto subquantities, of which one at least is used to check thegeneralization capability of the knowledge-based system.

[0006] The accuracy of the training and TDC detection in “operation” canbe raised by defining the course of the pressure inside the cylinderwith a time lag in relation to the TDC during the system training, sothat the TDC falls into the compression phase and the knowledge-basedsystem is trained for a TDC in this phase, and this deliberatesystematic error is then cancelled by the unchanged, trained,knowledge-based system. Furthermore TDC detection by the knowledge-basedsystem is improved if the electrical properties of the measuringarrangement, such as the filter frequency, when recording the pressurecurve inside the cylinder and the direct TDC time measurement on the onehand, and on the other hand the engine-specific parameters duringoperation, i.e. at TDC time detection in unknown operating states and/oruntrained machine-specific parameters, are made as identical aspossible.

[0007] Calculation of the cylinder output may be simplified by combiningthe pressure curve inside the cylinder with a linear scale in degrees ofcrank angle (°CA), referred to the TDC time determined from the pressurein the cylinder. If such a simplification is too inaccurate, the linearscale may be adapted to the shape of the actual curve over the timeinterval with the help of another knowledge-based system.

[0008] The meanings and scope of some of the terms employed in thesedocuments will now be defined:

[0009] “Pressure curve inside the cylinder” includes not only the directmeasurement of this variable but also other measurable variablesdepending on it, such as the elongation in a cylinder head bolt(EP-A-0170.256) or in the cylinder liner (EP-B-0671.618) and thepressure between cylinder head and nut (U.S. Pat. No. 5,179,857) or alength change between two walls of the cylinder head (EP-A-0175.449).

[0010] “Different operating points” are obtained by varying the revs perminute, fuel mixture, injection timing, ignition timing, load, chargingpressure and/or temperature for example, whereby the temperature isalways measured in the equilibrium state, i.e. after starting. Otheroperating states result from the number of operating hours logged by theparticular engine.

[0011] “Knowledge-based systems” are artificial neural networks (NN),neuro-fuzzy systems or fuzzy systems for instance. With fuzzy systemsthe process knowledge is put in selectively, whereas with neuro systemsthe system must acquire the process knowledge itself from the trainingphase. The “engine-specific parameters” include geometric dimensionslike cylinder bore and piston stroke, as well as compression andpossibly other characteristic data of an engine type.

[0012] The invention will now be described in more detail with referenceto a typical embodiment and the drawing.

[0013]FIG. 1 shows schematically an arrangement for direct measurementof the TDC by means of a capacitive sensor with the engine being turnedover.

[0014]FIG. 2 shows in similar presentation an arrangement for recordingmeasuring data, allocating a pressure value to each time or crank angleand storing these as correlated digital pairs.

[0015]FIG. 3 shows similarly again an arrangement for determining theTDC solely from the pressure inside the cylinder, using a trained,knowledge-based system, here a neural network (NN), and for calculatingthe cylinder output from the TDC obtained.

[0016]FIG. 4 plots the fluctuating rotational velocity ω of thecrankshaft around TDC during compression and after ignition. The crankangle is related to the TDC time.

[0017]FIG. 5 is the plot of a digital oscilloscope for a measuringseries with the arrangement in FIG. 1.

[0018]FIG. 6 is an enlarged detail from FIG. 5.

[0019]FIG. 7 is a similar plot of measurements recorded with thearrangement in FIG. 2.

[0020]FIG. 8 plots a number of TDC determinations in operating statesfor which the neural network had not been trained previously. Here againthe TDC detection is referred to the directly measured TDC as zero(FIGS. 1 and 5).

[0021] Verification of the new method is performed on a single-cylinderfour-stroke petrol (gasoline) engine, with spark ignition by coil. Itsbore measured 95 mm, stroke about 9 cm, and compression ratio about10:1.

[0022]FIG. 1 shows the engine schematically with a cylinder 2 in which apiston 3 is able to move. This is connected to a crankshaft 4, which atits right-hand end is fitted with a crank angle sensor or rotaryposition transducer 5. This is a commercially available device of theapplicant with the type designation 2612. The transducer 5 gives onesignal called the reference signal R at every revolution of the shaft 4.As second signal a signal E is generated in the rotary positiontransducer 5, which starting from a reference signal (°CA)—as the zeroso to speak—generates an angle mark in the form of a rectangular pulsefor each degree of crank angle (°CA), thus enhancing the measuringaccuracy in the event of speed fluctuations on shaft 4 during arevolution for example.

[0023] Fitted in the cylinder head is another sensor 7, which in FIG. 1is a capacitive sensor for direct TDC measuring. Its signal isdesignated TDC. This sensor 7 too is a product of the applicant andobtainable under the type designation 2629. It can be fitted only withpiston 3 turned over, otherwise ignition would destroy it.

[0024] All three signals R, E and TDC are led to a signal processing anddisplay unit 8 for instance, which in this example includes a commercialdigital cathode-ray oscilloscope CO and a computer PC. In this, toobtain high resolution and thus secure the necessary accuracy of thearrangement, the time signals E and R from the rotary positiontransducer and the TDC signal are scanned with a multiple, for examplesome ten thousand pulses per revolution of shaft 4. These individualpulses are subdivided in the oscilloscope CO and then transmitted to thecomputer PC, in which the allocation of TDC time from the TDC sensor tothe transducer signals E and R is performed. These measurements withsensor 7 while the engine 1 is turned over serve to calibrate the rotaryposition transducer 5 and the individual pulses derived from its timesignals E and R with regard to the TDC timing.

[0025] The various signals E, R and TDC are shown in FIGS. 5 and 6.Plotted horizontally in these diagrams is the time scale in degrees ofcrank angle (°CA). The bottommost curve in FIG. 5 represents the curveof the TDC signal measured by the sensor 7 in digitized individulvalues; the first maximum is to be allocated to the TDC of thecompression phase, and the second peak to the low-pressure TDC after theexhaust expulsion of the four-stroke engine.

[0026] The signal above this, appearing as a single pulse, is thereference signal R which indicates the zero at a revolution of the shaft4.

[0027] At the top in FIG. 5 is a solid band: the so-called incrementalsignal which is the mentioned angle or time signal E of the rotaryposition transducer 5. The enlargement or time stretching in FIG. 6shows the resolution of this band into the time signals E of thetransducer 5. Also plotted is the signal R and a detail from the TDCsignal.

[0028] In the arrangement according to FIG. 2, for recording measureddata to train and verify the generalization capability of the neuralnetwork (NN) 10 employed as knowledge-based system (FIG. 3), the sensor7 for direct TDC measurement is replaced by a pressure sensor 9, whichagain is a product obtainable from the applicant under the typedesignation 7061. To make the electrical conditions or properties duringtraining and use for the neural network as identical as possible, abandwidth limiter 11 with properties as identical as possible to thosein the target system is provided in the path for the pressure signal D,before the signal D reaches the oscilloscope CO. In the latter themeasured pressure curve is likewise resolved into digital individualvalues and stored with the associated values of the rotary positiontransducer signals E and R in a storage unit. The measured datarecording, value pairing and data set processing and storage areperformed for a number of different operating states; after a processingof the data pairs in the computer PC, for each pressure signal D onlythe TDC statement “yes” or “no” is stored in the form of digital values“1” or “0”. With TDC “yes” the time after TDC is designated, with TDC“no” the time before TDC. The TDC time in the data pair is thus exactlythe instant when the change from TDC “Yes” to TDC “no” takes place.

[0029] An example of such measured data recording is shown in FIG. 7.Here, as in FIG. 5, the pressure curve D, the reference signal R and theband of the time or increment signal E are plotted against time—again indegrees of crank angle (°CA).

[0030] Of the quantity of stored data sets from the measured datarecording, only a part is now used, embodying part of the measuredoperating states for training the neural network 10, which isaccomplished typically in the PC. The NN 10 employed may be described,not as a limitation but merely as an example probably capable of furtheroptimization. It is a so-called feedforward network with 70 inputneurons, two concealed or hidden layers of which the first has 6 and thesecond 15 neurons, and one output neuron. It is trained by the familiarback propagation learning method with a likewise familiar learningprogram (MatLab). The sigmoid function of the hyperbolic tangent servesas output or activation function for all neurons alike.

[0031] The training of network 10 is prolonged till the error of the TDCdetection is less than a preset limit for all operating statesconsidered.

[0032] The trained network 10 is now verified for its generalizationcapability. For this check the NN 10 is fed with the rest of the storeddata sets from the measured data recording, i.e. with data that have notbeen used for training purposes.

[0033] For every operating state not trained but measured in themeasured data recording, whose pressure signal and TDC value pairs havebeen stored, the stored pressure values are put onto the input neuron ofthe trained NN 10 but which is left unchanged after the training, sothat its output neuron then issues TDC signals detected from the valuesof the pressure signal D for each of these operating states, andcompares them with the associated stored pressure signal and TDC valuepair “yes” or “no”.

[0034] From FIG. 8, which shows the output function A of the NN 10related to the directly measured TDC as zero, it can be deduced that TDCcan be detected from the pressure signal D with an accuracy of 0.5° CAfor untrained operating states. This accuracy is adequate for manypurposes, though it can be enhanced further by further optimization ofthe system. From the measured curves in FIG. 8 for the output function Aof the unaltered, trained NN 10 for a few untrained operating states, itis evident that TDC is reached when the function value A=+0.9 is firstreached or exceeded.

[0035] As the pressure curve immediately around TDC is relativelystochastic, for the internal processing and allocation of the pressurevalues D and the incremental signals E the pressure signals D may bedelayed in the computer by a defined lag by means of a FIFO storage infamiliar manner, so that the TDC timing coincides with the steeper slopeof the compression phase. Needless to say, this systematic error iseliminated again during use after detecting the TDC, likewise in thecomputer.

[0036] In subsequent “operation” the measuring arrangement according toFIG. 3 serves TDC detection with unknown operating states. As soletransducer it contains the pressure sensor 9, which delivers thepressure signal D and leads it via the bandwidth limiter 11 which is asidentical as possible with the signal of the training data recording, tothe NN 10 unaltered after training which determines the TDC timing fromthe pressure curve for each operating state.

[0037] From the pressure signal D and the TDC detected with its help,the power output of the cylinder 1 may be calculated in the familiarmanner. This is done in a further calculating unit 12 of the measuringarrangement in FIG. 3; the unit 12 delivers the calculated output as thesignal P (FIG. 3).

[0038] In the simplest case a linear time scale in °CA is assumed, i.e.a constant angular velocity ω for the revolutions of the crankshaft 4,this linear scale being related to the “detected” TDC timing. As howeverthe measurement of the angular velocity ω (ordinate) plotted in FIG. 4against the crank angle CA related to TDC (abscissa TDC=0) shows, thissimplification is not quite correct because the angular velocity ω isretarded during the compression phase and accelerated after TDC, givingrise to an undulating curve. If therefore the simplifying assumptionshould not be admissible, with the help of a further knowledge-basedsystem the graduated scale can be adapted to the actual angular velocitycurve.

[0039] If direct TDC measurements and measured data recordings areperformed on different engine types having different engine-specificparameters (such as different geometric dimensions like piston strokeand cylinder bore, but also different compression and/or other varyingcharacteristic data), and these engine-specific variables are put intothe knowledge-based system via input neurons before training, togetherwith the internal pressure values D for various operating states, sothat these variables can be drawn upon for training, then the TDCdetection in the described example may be extended to detect the TDC onall engine types considered. Of course this requires more trainingeffort, but the method according to the invention is not renderedimpossible. In this way a knowledge-based system once trained can beused to determine the TDC or the cylinder power output of a variety ofengine types solely from the curve of the pressure inside the cylinder,for large-bore marine and generating engines, goods and passengervehicle engines, and even small engines as on lawn mowers.

REFERENCE LIST

[0040]1 Internal combustion engine

[0041]2 Cylinder

[0042]3 Piston

[0043]4 Crankshaft

[0044]5 Rotary position transducer, crank angle sensor

[0045]7 Capacitive TDC sensor

[0046]8 Signal processing and display unit

[0047]9 Pressure sensor

[0048]10 Neural network

[0049]11 Electrical “properties”

[0050]12 Power output calculation

[0051] A Output function of the neural network

[0052] D Signal for pressure inside the cylinder

[0053] E Digital incremental signal—or time signal of rotary positiontransducer

[0054] CO Digital oscilloscope

[0055] CA Crank angle

[0056] NN Knowledge-based system, in this example a neural network

[0057] TDC Top dead centre of piston stroke

[0058] P Cylinder power output

[0059] PC Computer

[0060] °CA Degrees of crank angle as time measure

[0061] ω Angular velocity of crankshaft

1. Method for exactly determining the top dead centre (TDC) of aninternal combustion engine (1) from the pressure curve inside thecylinder (D), characterized by detecting at various operating points onan engine (1) selected according to its specification the pressure curveinside the cylinder (D), and after direct measurement of the TDC time indegrees of crank angle (°CA), associated time values likewise in crankangle degrees (°CA) detected at the same time as digitized value pairs,correlated and stored in a data set. Furthermore the pressure inside thecylinder (D) is if necessary put into a knowledge-based system (NN 10)together with engine-specific parameters as input variables, its outputgiving a certain value for the TDC time which then trains the system (NN10) with elements from the data set till it recognizes from the pressureinside the cylinder (D) the associated TDC time of the directmeasurement at all operating points used for training purposes, also ifnecessary for different engine-specific parameters, with the requiredaccuracy, and finally determining from the measured pressure in thecylinder (D) at operating points not trained, with differentengine-specific parameters if necessary, the TDC time with the help ofan unaltered but trained, knowledge-based system (NN 10).
 2. Methodaccording to claim 1 , characterized by the curve of the pressure insidethe cylinder (D) being defined with a time lag compared with the TDCtime defined during the training of the system (NN 10) and used by thetrained system (NN 10) for TDC detection.
 3. Method according to one ofclaims 1 or 2, characterized by the pressure curve inside the cylinder(D) being defined with a time lag compared with the TDC time whiletraining the system (NN), and moreover this deliberate systematic errorbeing cancelled during operation with the unaltered, trained,knowledge-based system (NN 10).
 4. Method according to one of claims 1to 3 , characterized by the electrical properties (11) of the measuringarrangement being as identical as possible when detecting the pressureinside the cylinder (D) and at the direct TDC time measurement on theone hand, and during operation with knowledge-based systems (NN 10) onthe other hand.
 5. Method according to one of claims 1 to 4 ,characterized by combining the pressure curve inside the cylinder (D)with a scale in degrees of crank angle (°CA) related to the TDC timedetermined from the pressure inside the cylinder (D), for calculatingthe power in the cylinder (1).